What type of affordable and green heating for historical building?

This is a difficult one. First we need to know what state this is in, to determine what the winter design temp. is recommended.Second, is there an attic space? I ume not because of the square feet total you gave.So you see my dilemma. If 400 sq. ft. is a true total for this space including up & downstairs, the only efficient way to heat this space is with electric baseboard heat. Ideally, you put a heater under a window so that the heat will rise to block the heat loss in winter. If windows are sparse, a wall space will do, Electric baseboard heaters are about 2 inches wide & 6 inches tall. They can be as short as 2 feet & up to 6 feet long. A home as small as this one should not require more than 10 kw (34,410 btu) to heat. The total cost to install this type of heat is far less than a complete gas furnace system. It is accepted in the "green" sense. These can be purchased in decorator colors or in any color to match the walls.